Corona Rings/AL alloy in Power/Current Transformer/Surge Arrestor/HVBushing/Gas Insulated Switchgear

Описание: the difference between a grading ring and a corona ring?
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A grading ring is used to ensure a uniform voltage distribution along the length of an electrical device. This is important for surge arresters so each MOV disc in the arrester is energized at the appropriate voltage.

A corona ring is traditionally used to electrically shield external hardware to prevent corona from developing. This corona could lead to degradation of insulating materials or create interference to electronic communication.

Surge arresters below 500 kV system voltage do not typically need corona rings.

Why does Grading ring use in the Transmission line?
Grading ring always provides with the insulator in the transmission line. The Design of the Grading rings is similar to corona rings and they encircle insulators of the Transmission line. The potential gradient or electric fields across the line insulator is not uniform. The highest gradient observed at the endpoint insulators towards the high voltage conductor. Due to this continuous grading at high voltage, the insulators near the high voltage conductor are easily deteriorated and breakdown. The line observes short circuit earth fault because of the breakdown insulator.

grading ring in transmission line

It reduces the potential gradient along the insulator and prevents electrical insulator breakdown. The grading ring covers the surrounds ends of the insulator near the high voltage side and creates a uniform distribution of voltage and reduces the stress on the insulator.
The grading ring reduces the aging effect and deterioration of the line insulator.
During high voltage conditions, it prevents the insulator from the breakdown.
It also serves the purpose of the corona ring and prevents corona discharge of the line.
The grading ring is electrically connected with the conductor. The Ring shape grading rings are used in the insulator of the transmission line and the surface of the ring is very smooth for uniform distribution of the field.

Design of Grading Ring
The design of the grading ring depends on the following factors

Rated voltage of the Transmission line
Maximum over voltage rating.
Type of transmission line insulator
Weather condition
Testing
Tests for checking the quality of Grading Ring before use in Transmission Line,

Mechanical Strength,
Resistance
Smoothness of the ring surface.

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